I was reading about telephone lightning arrestors in my ARRL RFI Book (first edition, copyright 1998), in chapter 9 "Telephone RFI" (by Pete Krieger, previously WA8KZH, now K8COM) and on page 9.6, the following is stated (emphasis mine):
The Lightning Arrestor
Telephone service enters a house at a grounded, fused lightning arrestor located outside at the house end of the telephone company drop. Years of exposure to weather or moisture can cause corrosion or discoloration (onset of corrosion) of wires, junction boxes or components inside the lightning protector housing. If the arrestor is accessible, a good visual inspection may reveal potential problems. Lightning arrestors, especially those that have done their job a few times, can become nonlinear, acting like a diode, rectifying any RF energy present on the phone lines (just like the crystal radios many hams built in their early years). If you discover that a lightning arrestor is creating RFI, the fact that it's rectifying RF is one indication that it needed to be replaced anyway! Modern arrestors are less prone to RFI problems than older ones.
Just a few notes about this quote... This is the entire (small) section, so this is likely all the information that I have behind my question. Also, the context does indicate that the lightning arrestor is standard telephone service equipment, and therefore part of a standard land-line telephone installation; therefore, this question may be limited to only standard telephone company equipment. Quite possibly, it may be a broader generalization -- I just don't know.
So, it's that last sentence that piqued my interest. Why, exactly, are modern lightning arrestors less prone to RFI problems? Is it more even "wearing" so it doesn't become a diode? Is it a better design, so it lasts longer and fails more gracefully?
I emphasized the "diode" aspects of the quote from the book a little more in preparation for making the following request:
I would like to argue that for me to accept an answer, your answer has to show either how semiconducting happened more frequently with the old style, or alternately show how semiconducting happens less frequently with the new style. This is because I can't really decide between the two really great answers already given. How did a lightning hit (or moisture, or time) cause carbon blocks (the old style) to semiconduct?